Usually, a wheel bearing apparatus is used to freely rotationally support a wheel hub that mounts the wheel, via a rolling bearing, for driving wheels and driven wheels. For structural reasons, an inner ring rotation type is generally adopted for a driving wheel and both an inner ring rotation and outer ring rotation types are adopted for a driven wheel. Double row angular contact ball bearings are widely used in such a bearing apparatus. Reasons for this is that they have a desirable bearing rigidity, high durability against misalignment and small rotation torque to improve the fuel consumption. The double row angular contact ball bearing has a structure where a plurality of balls is interposed between a stationary ring and a rotational ring. A predetermined contacting angle is applied to the balls relative to the stationary and rotational rings.
The bearing apparatus for a wheel of a vehicle is broadly classified into a first-fourth generation structure. In a first generation type, a wheel bearing with a double row angular contact ball bearing is fit between a knuckle forming part of a suspension and a wheel hub. In a second generation structure, a body mounting flange or a wheel mounting flange is directly formed on the outer circumference of an outer member. In a third generation structure, one of the inner raceway surfaces is directly formed on the outer circumference of the wheel hub. In a fourth generation structure, the inner raceway surfaces are directly formed on the outer circumference of the wheel hub and the constant velocity universal joint.
In prior art wheel bearing apparatus formed with a double row rolling bearing, since both bearing row arrangements are the same in the double row bearing, the apparatus has a sufficient rigidity during straight way running, however, optimum rigidity cannot always be obtained during curved way running. The positional relationship between the wheels and the bearing apparatus is usually designed so that the weight of the vehicle acts at substantially the center between the rows of bearing balls during the straight way running. However, a larger radial load and a larger axial load are applied to vehicle axles on the side opposite to the curving direction (i.e. axles of the left hand side of vehicle when right hand curving). Accordingly, it is effective to have a larger rigidity on the bearing row of the outer-side than that of the bearing row of the inner-side in order to improve the durability and strength of the bearing apparatus. Thus, a known vehicle wheel bearing apparatus is shown in FIG. 3 that can have a high rigidity without enlargement of the bearing apparatus. In the description below, the term “outer-side” (left hand side in the drawings) of the apparatus denotes a side that is positioned outside of the vehicle body. The term “inner-side” (right hand side in the drawings) of the apparatus denotes a side that is positioned inside of the body when the bearing apparatus is mounted on the vehicle body.
The vehicle wheel bearing apparatus 50 is formed with a double row angular contact ball bearing including an outer member 51 integrally formed on its outer circumference with a body mounting flange 51c. The flange 51c is mounted on a knuckle (not shown) of a vehicle. Its inner circumference includes double row outer raceway surfaces 51a, 51b. An inner member 55 includes a wheel hub 52 with a wheel mounting flange 53 integrally formed at one end for mounting a wheel (not shown). One inner raceway surface 52a is formed on the outer circumference of the wheel hub opposite to one 51a of the double row outer raceway surfaces 51a, 51b. A cylindrical portion 52b axially extends from the inner raceway surface 52a. An inner ring 54 is fit onto the cylindrical portion 52b. The inner ring includes an outer circumference with the other inner raceway surface 54a opposite to the other raceway surface 51b of the double row outer raceway surfaces 51a, 51b. Double row balls 56, 57 are freely rollably contained between the outer raceway surfaces 51a, 51b and inner raceway surfaces 52a, 54a of the inner member 55. Cages 58, 59 rollably hold the balls 56, 57.
The inner ring 54 is axially immovably secured by a caulked portion 52c. The caulked portion 52c is formed by radially outwardly plastically deforming the cylindrical portion 52b of the wheel hub 52. Seals 60, 61 are mounted in annular openings formed between the outer member 51 and the inner member 55. The seals 60, 61 prevent leakage of grease contained within the bearing apparatus and the entry of rain water or dusts into the bearing apparatus from the outside.
A pitch circle diameter D1 of the outer-side ball group 56 is set larger than a pitch circle diameter D2 of the inner-side ball group 57. Accordingly, the diameter of the inner raceway surface 52a of the wheel hub 52 is larger than the diameter of the inner raceway surface 54a of the inner ring 54. The outer raceway surface 51a of the outer-side of the outer member 51 is larger than that of the outer raceway surface 51b of the inner-side of the outer member 51. Also, the number of outer-side balls 56 is larger than the number of the inner-side balls 57. By setting the pitch circle diameter D1 of the outer-side larger than the pitch circle diameter D2 of the inner-side (D1>D2), it is possible to obtain a large rigidity of the bearing apparatus 50. Thus, this extends the life of the bearing. (Japanese Laid-open Patent Publication No. 108449/2004).
In the prior art wheel bearing apparatus 50, the pitch circle diameter D1 of the outer-side ball group 56 is set larger than the pitch circle diameter D2 of the inner-side ball group 57. Accordingly, the diameter of the outer-side outer raceway surface 51a of the outer member 51 is larger than that of the inner-side outer raceway surface 51b of the outer member 51. Additionally, the number of the outer-side balls 56 is larger than the number of the inner-side balls 57. This improves the rigidity of the outer-side bearing row and thus extends the life of the wheel bearing apparatus 50. However, to the contrary, since the bearing life of the inner-side ball group 57 becomes disadvantageous as compared with the outer-side ball group 56, it has been desired to solve this problem and to provide a wheel bearing apparatus for a vehicle that can simultaneously achieve reduction of its size and weight and increase its strength and rigidity to improve its durability.